Remediation of metal-bearing aqueous waste streams via direct carbonation

Direct carbonation using liquid carbon dioxide can be used for the remediation: of aqueous streams or slurries while sequestering carbon dioxide in the form of metal carbonates. Carbonic acid (pH = 2.9), which formed when the aqueous phase was contacted with excess liquid carbon dioxide at ambient temperature (295 K) and elevated pressure (6.89 MPa), reacted with the metal cations to form metal carbonates in the agitated vessel. These metal carbonates precipitated out of solution as the pH returned to neutral when the system was depressurized. Electric are furnace K061 dust, red mud from the Bayer process of alumina manufacture, and metal-bearing wastewater streams were amenable to this treatment. The treatment of a K061-dust slurry from a steel plant was semi-continuous. The dust particles were retained in the high-pressure reaction vessel as fresh water was continuously injected and high-pressure, metal carbonate-bearing water was withdrawn. The water residence time in the reactor was 12 min. About 30% of the metal in the K061 dust was extracted into a metal carbonate product, and 98% of the metal in the carbonate product was zinc. Unfortunately, lead was not selectively extracted from the dust. Red mud was neutralized in batch experiments that lasted 5-15 min. The pH of a 45 wt % red mud/55 wt % water slurry was reduced from 12.5 to 7. A post-treatment pH elevation to 9.5 was attributed to slow desilication reactions that occurred over 1-2 weeks at ambient temperature and pressure. A plating bath wastewater stream containing aluminum (666 ppm) and zinc (40 ppm) was contacted with excess liquid carbon dioxide for 5 min. The aluminum and zinc concentrations were reduced by 89% and 90%, respectively, and the metal carbonate precipitate was easily filtered. Although the combined sequestration potential of these wastes is small, the ability to effectively remediate waste streams could lead to an industrial interest in the development of direct carbonation technology.